The present invention relates to a pneumatic tire, and particularly relates to a pneumatic radial tire for passenger cars having an improved puncture resistance. (2) Description of the Prior Art:
Recently, travelling speed of tires has been increased due to the improved paved roads, high speed roads, etc. and to the improvement of performance of vehicles. Radial tires have recently been used as a tire having improved high speed performance and cornering performance, and stability at high speed. Hence, countermeasures against puncture of radial tires effective a difficult problem for many years in the tire manufacture field, still remain.
Troubles of tires due to puncture have recently been noticeably decreased due to improved paved roads, but the puncture on a high speed road or on a busy traffic area is very dangerous, and the development of tires, which are safe in running, is very important in the tire manufacture field. A tire, which is resistant to puncture comprises a rubberized belt layer containing metal cords embedded therein and has been utilized. Further, a tire having an improved puncture resistance has also been proposed, the tire being a combination of a tire provided on its inner surface with a puncture-sealing layer formed of a viscous rubber composition and a tire having the above described metal cord-containing belt layer.
Other various methods of preventing puncture of tire have been proposed, for example, in Japanese Patent Application Publication No. 6,934/51, No. 5,288/60 and No. 39,453/75 and Japanese Patent Laid Open Application No. 52,704/75 and No. 21,402/75. The tires disclosed in these methods are provided at the inner surface of their crown portion with a viscous rubber puncture-sealing layer (The term "viscous rubber puncture-sealing layer" means a puncture-sealing layer formed of a viscous rubber composition). Such tires have been improved to such an extent that about 70% of punctures at the crown portion due to nail, broken glass piece, screw, etc. are prevented. However, it has been found that such kind of tires have the above described merits, but also have the following defects. That is, there are the following two kinds of punctures.
In a first kind of puncture, which occurs at the contact surface of crown portion with the ground, the the nails, broken glass piece, screws, etc. penetrate into the inner surface of a tire to induce leakage of air, thus producing puncture of the tire. This compresses about 94% of all punctures.
In the second kind of puncture which occurs at the sidewall portion, the number of occurrences of punctures is small, but the puncture occurs when a sidewall portion is subjected to external scratches by sharp stone, rock, etc. The external scratches produced at the sidewall portion lead to bursting of the tire, and hence are very dangerous.
The second kind of puncture is due to the property inherent to radial tires. That is, the sidewall portion of radial tire greatly deforms and is more flexible than the sidewall portion of bias tire. Therefore, punctures started from the sidewall portion occur often in radial tire. Tires provided with a puncture-sealing layer formed of the above described viscous rubber composition on the whole circumference of the inner surface of the crown portion are very effective for preventing the first kind of puncture, but are not effective for preventing the second kind of puncture. When such tires are used, the total number of punctures is smaller than those of conventional tires, but the number of punctures started from the sidewall portion is apt to increase.
The inventors have intended to provide a safety pneumatic radial tire which is free from the above described various drawbacks and has an improved puncture resistance at both of the contact surface of crown portion with ground and the sidewall portion.
That is, the inventors have considered that it is necessary to improve the puncture resistance at the contact surface of crown portion with ground as in the case of conventional tires and at the same time improve the puncture resistance against sharp stone and rock at the sidewall portion. Various investigations have shown that the improvement of puncture resistance at the crown portion in the present invention does not lie in the provision of a completely puncture-free tire, but lies in that, when nail and the like penetrate through the crown portion of a tubeless tire, leakage of air from the surroundings of the nail and the like is prevented, and the sudden leakage of air through the hole formed after the nail and the like have been fallen out is also prevented, whereby the safely travelling state of the tubeless tire is secured. However, it is impossible to seal completely the leakage of air due to the kind of foreign substances, such as nails and the like, the penetrated state thereof, the moving of puncture-sealing layer caused by the heat accumulation during the running and the shape of tire. Also, the tire sometimes continues to run under such a state that air pressure in the tire is lower than the defined air pressure of initially set air pressure. As the result, it has been found that the sidewall portion deforms more and more and the scratches at the sidewall portion due to sharp stone and rocks are apt to increase. That is, the effect of improving the puncture resistance is not fully developed. Further, an experiment shows that, although a conventional puncture-sealing layer is effective for the puncture resistance at the crown portion, the layer often moves to the middle portion of crown portion due to the heat accumulation and centrifugal force during the running of tire and the puncture resistance thereof is decreased, and the balance of the tire is lost.
The inventors, based on the above described phenomena, have investigated the proper inner shape of crown portion, on which a puncture-sealing layer is arranged, as a means for preventing the moving of the layer, and proper shape and position of a protection rib as a means for preventing puncture starting from the sidewall portion.